This invention relates to analog to digital conversion systems and more particularly to analog to digital conversion systems required to provide digitized samples of an analog signal over a wide range of selectable rates.
As is known in the art, various systems, such as radar systems, require, in application where digital signal processors are used to process radar return signals, conversion of the analog radar returns signals into corresponding digital signals. For example, radar signal processors are used for determining the range and/or relative velocity, or velocities, associated with one, or more, objects. Radar pulses are transmitted at a rate referred to as the pulse repetition frequency (PRF). During a predetermined time after pulse transmission, i.e., the expected time of arrival of the pulses, radar return signals are sampled. Based on the difference in time between the time of pulse transmission and the time at which the sample is taken, each one of the samples would correspond to a range, or distance between the radar system antenna and the object producing the sampled return. The rate at which the samples are taken following each transmitted pulse is directly related to the desired range resolution. The process is referred to as range gating, where each time a sample is taken represents a range cell, or gate, of the return produced by the object at the range corresponding to the time the sample is taken. As is also known in the art, in some applications it is necessary to have the radar system operate over a broad range of pulse repetition frequencies. Further, in some applications, it is necessary to have the samples taken at many different sampling rates for a broad range of range resolutions. One system used to produce the digitized samples includes an analog to digital converter. With such converter, a sample of the video signal is taken and converted into a corresponding digital sample in response to a sampling signal. In order to accommodate the variation in sampling/conversion rates, the analog to digital converter has to be capable of operating over this relatively large range of sampling/conversion rates.